Development of hybrid plant breeding has made possible considerable advances in quality and quantity of crops produced. Increased yield and combination of desirable characteristics, such as resistance to disease and insects, heat and drought tolerance, along with variations in plant composition are all possible because of hybridization procedures. These procedures frequently rely heavily on providing for a male parent contributing pollen to a female parent to produce the resulting hybrid.
Field crops are bred through techniques that take advantage of the plant's method of pollination. A plant is self-pollinating if pollen from one flower is transferred to the same or another flower of the same plant. A plant is cross-pollinated if the pollen comes from a flower on a different plant.
In Brassica, the plant is normally self-sterile and can only be cross-pollinated. In self-pollinating species, such as soybeans and cotton, the male and female plants are anatomically juxtaposed. During natural pollination, the male reproductive organs of a given flower pollinate the female reproductive organs of the same flower.
Maize plants (Zea mays L.) present a unique situation in that they can be bred by both self-pollination and cross-pollination techniques. Maize has male flowers, located on the tassel, and female flowers, located on the ear, on the same plant. It can self or cross pollinate. Natural pollination occurs in maize when wind blows pollen from the tassels to the silks that protrude from the tops of the incipient ears.
A reliable method of controlling fertility in plants would offer the opportunity for improved plant breeding. This is especially true for development of maize hybrids, which relies upon some sort of male sterility system and where a female sterility system would reduce production costs.
The development of maize hybrids requires the development of homozygous inbred lines, the crossing of these lines, and the evaluation of the crosses. Pedigree breeding and recurrent selection are two of the breeding methods used to develop inbred lines from populations. Breeding programs combine desirable traits from two or more inbred lines or various broad-based sources into breeding pools from which new inbred lines are developed by selfing and selection of desired phenotypes. A hybrid maize variety is the cross of two such inbred lines, each of which may have one or more desirable characteristics lacked by the other or which complement the other. The new inbreds are crossed with other inbred lines and the hybrids from these crosses are evaluated to determine which have commercial potential. The hybrid progeny of the first generation is designated F1. In the development of hybrids only the F1 hybrid plants are sought. The F1 hybrid is more vigorous than its inbred parents. This hybrid vigor, or heterosis, can be manifested in many ways, including increased vegetative growth and increased yield.
Hybrid maize seed can be produced by a male sterility system incorporating manual detasseling. To produce hybrid seed, the male tassel is removed from the growing female inbred parent, which can be planted in various alternating row patterns with the male inbred parent. Consequently, providing that there is sufficient isolation from sources of foreign maize pollen, the ears of the female inbred will be fertilized only with pollen from the male inbred. The resulting seed is therefore hybrid (F1) and will form hybrid plants.
Environmental variation in plant development can result in plants tasseling after manual detasseling of the female parent is completed. Or, a detasseler might not completely remove the tassel of a female inbred plant. In any event, the result is that the female plant will successfully shed pollen and some female plants will be self-pollinated. This will result in seed of the female inbred being harvested along with the hybrid seed which is normally produced. Female inbred seed is not as productive as F1 seed. In addition, the presence of female inbred seed can represent a germplasm security risk for the company producing the hybrid.
Alternatively, the female inbred can be mechanically detasseled by machine. Mechanical detasseling is approximately as reliable as hand detasseling, but is faster and less costly. However, most detasseling machines produce more damage to the plants than hand detasseling. Thus, no form of detasseling is presently entirely satisfactory, and a need continues to exist for alternatives which further reduce production costs and to eliminate self-pollination of the female parent in the production of hybrid seed.
A reliable system of genetic male sterility would provide advantages. The laborious detasseling process can be avoided in some genotypes by using cytoplasmic male-sterile (CMS) inbreds. In the absence of a fertility restorer gene, plants of a CMS inbred are male sterile as a result of factors resulting from the cytoplasmic, as opposed to the nuclear, genome. Thus, this characteristic is inherited exclusively through the female parent in maize plants, since only the female provides cytoplasm to the fertilized seed. CMS plants are fertilized with pollen from another inbred that is not male-sterile. Pollen from the second inbred may or may not contribute genes that make the hybrid plants male-fertile. Usually seed from detasseled normal maize and CMS produced seed of the same hybrid must be blended to insure that adequate pollen loads are available for fertilization when the hybrid plants are grown and to insure cytoplasmic diversity.
There can be other drawbacks to CMS. One is an historically observed association of a specific variant of CMS with susceptibility to certain crop diseases. This problem has discouraged widespread use of that CMS variant in producing hybrid maize and has had a negative impact on the use of CMS in maize in general.
One type of genetic sterility is disclosed in U.S. Pat. Nos. 4,654,465 and 4,727,219 to Brar, et al. However, this form of genetic male sterility requires maintenance of multiple mutant genes at separate locations within the genome and requires a complex marker system to track the genes and make use of the system convenient. Patterson also described a genic system of chromosomal translocations which can be effective, but which are complicated. (See, U.S. Pat. Nos. 3,861,709 and 3,710,511.)
Many other attempts have been made to improve on these drawbacks. For example, Fabijanski, et al., developed several methods of causing male sterility in plants (see EPO 89/3010153.8 publication no. 329,308 and PCT application PCT/CA90/00037 published as WO 90/08828). One method includes delivering into the plant a gene encoding a cytotoxic substance associated with a male tissue specific promoter. Another involves an antisense system in which a gene critical to fertility is identified and an antisense to the gene inserted in the plant. Mariani, et al. also shows several cytotoxic antisense systems. See EP 89/401, 194. Still other systems use “repressor” genes which inhibit the expression of another gene critical to male sterility. PCT/GB90/00102, published as WO 90/08829.
A still further improvement of this system is one described at U.S. Pat. No. 5,478,369 in which a method of imparting controllable male sterility is achieved by silencing a gene native to the plant that is critical for male fertility and replacing the native DNA with the gene critical to male fertility linked to an inducible promoter controlling expression of the gene. The plant is thus constitutively sterile, becoming fertile only when the promoter is induced and its attached male fertility gene is expressed.
In a number of circumstances, a male sterility plant trait is expressed by maintenance of a homozygous recessive condition. Difficulties arise in maintaining the homozygous condition, when a transgenic restoration gene must be used for maintenance. For example, a natural mutation in a gene critical to male sterility can impart a male sterility phenotype to plants when this mutant allele is in the homozygous state. This sterility can be restored when the non-mutant form of the gene is introduced into the plant. However, this form of restoration removes the desired homozygous recessive condition, restores full male fertility and prevents maintenance of pure male sterile maternal lines.
This issue can be avoided where production of pollen containing the restoration gene is eliminated, thus providing a maintainer plant producing only pollen not containing the restoration gene, and the progeny retains the homozygous condition. An example of one approach is shown in Dellaporta et al., 6,743,968, in which a plant is produced having a hemizygotic construct comprising a gene that produces a product fatal to a cell, linked with a pollen-specific promoter, and the restoration gene. When crossed with the homozygous recessive male sterile plant, the progeny thus retains the homozygous recessive condition.
As noted, an essential aspect of much of the work underway with male sterility systems is the identification of genes impacting male fertility. Such a gene can be used in a variety of systems to control male fertility including those described herein. Previously, a male fertility gene has been identified in Arabidopsis thaliana and used to produce a male sterile plant. Aarts, et al., “Transposon Tagging of a Male Sterility Gene in Arabidopsis”, Nature, 363:715-717 (Jun. 24, 1993). U.S. Pat. No. 5,478,369 discloses therein one such gene impacting male fertility. In the present invention the inventors provide novel DNA molecules and the amino acid sequence encoded that are critical to male fertility in plants. These can be used in any of the systems where control of fertility is useful, including those described above.
Thus, one object of the invention is to provide a nucleic acid sequence, the expression of which is critical to male fertility in plants.
Another object of the invention is to provide a DNA molecule encoding an amino acid sequence, the expression of which is critical to male fertility in plants.
Yet another object of the invention is to provide a promoter of such nucleotide sequence and its essential sequences.
A further object of the invention is to provide a method of using such DNA molecules to mediate male fertility in plants.
Further objects of the invention will become apparent in the description and claims that follow.